Tetramorium hispidum, along with its sister species T. spinosum, presents an unsolved evolutionary mystery of the North American ant fauna. Tetramorium, an ubiquitous and extraordinarily diverse genus in Africa and Asia, is nearly absent in the Americas. These two species, both confined to the desert Southwest, are the only native Tetramorium in the hemisphere. When and how the ancestor of these ants arrived is not known. Tucson, Arizona, USA

Trachymyrmex desertorum workers in the fungus garden. Tucson, Arizona, USA